To determine the limits to oxidation of exogenous glucose by skeletal muscle, the effects of euglycaemia (plasma glucose 5 mM, ET) and hyperglycaemia (plasma glucose 10 mM, HT) on fuel substrate kinetics were evaluated in 12 trained subjects cycling at 70% of maximal oxygen uptake (VO2, max) for 2 h. During exercise, subjects ingested water labelled with traces of U-14C-glucose so that the rates of plasma glucose oxidation (Rox) could be determined from plasma 14C-glucose and expired 14CO2 radioactivities, and respiratory gas exchange. Simultaneously, 2-3H-glucose was infused at a constant rate to estimate rates of endogenous glucose turnover (Ra), while unlabelled glucose (25% dextrose) was infused to maintain plasma glucose concentration at either 5 or 10 mM. During ET, endogenous liver glucose Ra (total Ra minus the rate of infusion) declined from 22.4 +/- 4.9 to 6.5 +/- 1.4 mumol/min per kg fat-free mass [FFM] (P < 0.05) and during HT it was completely suppressed. In contrast, Rox increased to 152 +/- 21 and 61 +/- 10 mumol/min per kg FFM at the end of HT and ET respectively (P < 0.05). HT (i.e., plasma glucose 10 mM) and hyperinsulinaemia (24.5 +/- 0.9 microU/ml) also increased total carbohydrate oxidation from 203 +/- 7 (ET) to 310 +/- 3 mumol/min per kg FFM (P < 0.0001) and suppressed fat oxidation from 51 +/- 3 (ET) to 18 +/- 2 mumol/min per kg FFM (P < 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)